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Evanescent Wave Optical-Fiber Sensing (Temperature, Relative Humidity, and pH Sensors) Ainhoa Gaston, Ibon Lozano, Fátima Perez, Fernando Auza, and Joaquín Sevilla
Abstract—Sensitive and versatile evanescent wave-sensing systems featuring polished optical fiber-based sensor designs with low-cost light sources have been developed for temperature, relative humidity, and pH measurements. The work herein contained describes the fabrication of three types of sensors based on standard silica, single-mode fibers previously subjected to a lateral polishing of the cladding. Temperature sensing through oils whose refractive index varied linearly with temperature showed applicability with up to 5 dB/ C for a 5 range. Polyvinyl alcohol films on the fibers showed almost 10-dB linear variation from 70% to 90% relative humidity. Sol-gel trapped dyes as thin films on the polished surface were capable of performing 15-dB output variation (although not linearly) for pH ranging from 2 to 11.
I. INTRODUCTION
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OUTINE analytical measurements, both physical and chemical, are important for the monitoring of processes throughout industrial production. Sensors, thus, represent a determinant sector of the engineering market activity. The development of optical sensors and fiber-optic sensors, in particular, has experimented a revolution over the last two decades with the improvement of the characteristics and the proliferation of sensing applications [1], [2]. Advantages over commercially available conventional sensors do not overcome the complexity and the cost of the supporting systems yet, and many devices are still relegated to research level development. In order to compete with other existing devices, key parameters such as cost, size, and sensitivity should be jealously attended from the mere applicability viewpoint. Optical fibers, which were designed to be successful waveguides, have been converted into sensors and many other devices (couplers, filters, and modulators) by slight induced perturbation of their structure. The side polishing of optical fibers has been extensively studied and reported [3] and it is a conventional method to access the light propagated in an optical fiber. The cylindrical symmetry of the structure of the fiber is altered when the fiber is polished in a longitudinal plane, proportioning a very sensitive region within the fiber. The intensity of the propagating electromagnetic field can be perturbed by the external medium lying onto the polished surface due to the penetration of the evanescent field into that medium. The propagated light output turned sensitive to the refractive index of the neighboring medium, and with the selection of sensing Manuscript received February 15, 2002; revised June 30, 2003. This work was supported in part by the Spanish Government (TIC 98/397-CO3-01) and the local Government of Navarra. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna Grazia Mignani. The authors are with the Department of Electrical and Electronic Engineering, Public University of Navarra, Pamplona, Spain (e-mail:
[email protected]). Digital Object Identifier 10.1109/JSEN.2003.820349
materials to coat the interaction section, it is possible to make the fiber system sensitive to diverse physical measurable magnitudes or measurands. Hence, the design of specific fiber-optic sensors should search overlay materials which could undergo refractive index changes as a response to changes in the measurand. This work pretends to illustrate a fabrication technique of samples of side-polished optical fibers (developed aiming to obtain a fabrication procedure well suited for industrialization a with cheap materials) and their conversion, using specific overlay materials, into three types of sensors of industrial interest, such as temperature, relative humidity in air, and pH of aqueous solutions. II. FABRICATION
OF SIDE-POLISHED SINGLE-MODE OPTICAL FIBERS
The polishing method here presented was developed trying to find a procedure simpler than the ones reported in the literature [3], [4], and using cheap materials. The first stage in order to polish a fiber is to fix it in a solid host-block that allows manipulation and protection during the polishing. In our case, the blocks were made of polymeric commercial resins, which were applied in successive mouldings. The optical fiber used in all the experiments was silica 9/125 m, one of the standards in telecommunications cables (SMF28 from Optical Cables, step index of 0,0041 and cut-off wavelength 1270 nm). In the first stage, the length of fiber to be polished was placed in a concave mould (with an estimated curvature radius of 2.25 cm) and it was covered with epoxy resin (Epo-Thin from BUEHLER). Once it was cured (more than 20 h), the piece was then introduced in another mould that allowed the exit of the ends of the fiber in a convenient manner for the handling and the later splicing with standard connectors. For the second stage, the moulding was not that critical because the region of interest was already embedded in epoxy, therefore an acrylic resin with a much quicker hardening (several minutes) process was used. The polishing was carried out in a commercial lapping ma) as abrachine from BUEHLER using aluminum oxide ( sive. The process was repeated employing abrasive powder of decreasing grain size, starting with 9.5 m, then 3 m and to end with 1 m, which assured a mirror-like finish of the polished surface [5]. The surface was cleaned in a water ultrasound bath. The whole process was controlled by direct observation of the polished region with an optical microscope; even erosion depth control was made off-line by microscopic observation until the core vicinity was reached. The final stages of the polishing and the determination of the ultimate polished depth were done by the liquid-drop method (also known as the oil-drop test) [6]. Op-
1530-437X/03$17.00 © 2003 IEEE
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tical microscopy can be used to trace a depth profile (with significant uncertainty in the absolute values), while the liquid-drop method allows a more rigorous determination of the minimum distance from the fiber core to the polished surface. In some cases, surface profilometry was used for further characterization of polished surfaces. The samples better suited for sensing were polished very closed to the core-cladding interface (between 3 and 6 m), and presented an exposed length between 2 and 3 mm. Finally, in order to obtain sensor prototypes, several materials were applied onto the polished fibers depending on the particular application and desired type of sensor. For the sensor model with vegetable oils emulating semi-infinite media, a small quantity of liquid was held in a convenient cell. For the other two examples (the moisture sensor and the pH approach), a thin solid film was deposited.
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Fig. 1. Optical power loss of the polished-fiber sample (referred to the value obtained in air) when it is in contact with two different oils (olive and sunflower) against oil temperature variation. The wavelength used was 1550 nm.
III. RESULTS A. Temperature Sensor The temperature sensor prototypes are constructed by mounting a small deposit over the polished zone of the fiber able to retain certain amount of liquid. As the fiber is fixed in a flat surface of the resin block, this small chamber was constructed placing a rubber O-ring on this surface and, after filling with the liquid, closing the system with a piece of glass (a microscope slide). In order to obtain an alternative value of the temperature, needed for calibration, a thermocouple was placed in contact with the liquid inside the chamber. The different pieces composing the system were stuck together with the aid of clumps, without any glue, so that it can be dismounted and the liquid changed. Therefore, different oils can be tested over the same polished fiber. As previously stated, the liquids used to build these sensors were in all cases vegetable oils of commercial grade (bought directly from the supermarket). The whole system was then exposed to temperature changes by blowing onto it with a hair air dryer or, more typically, by introducing it in a thermal bath. The transmission of optical power along a polished fiber, which was in contact with the vegetable oils, was monitored as the temperature of the oils was elevated. Fig. 1 shows the curves of attenuation in optical power versus temperature obtained for a prototype using the same polished fiber and olive and sunflower oils. The experimental error is in the order of dots size in the plot: dB for power attenuation and C for temperature. Light sources and detectors used showed stability through typical experiments duration much better than that exhibited by the couplers. Therefore blank power control was accomplished only before and after the experiment. The use of oil for sensing temperature has already been reported [7], [8], but in conjunction with the extreme sensitivity of the single mode polished fibers, performance obtained is clearly improved. Once the general lines of the system response to temperature have been outlined, specific sensors can be tailored for different applications selecting appropriate oils and ranges. Fig. 2 shows intervals of five degrees where linearity is very good and sensitivity up to 5 dB/ C.
Fig. 2. Calibration curve of temperature prototypes. Data from Fig. 1 for particular ranges.
Another relevant feature of the proposed sensor is the response time, which can be very fast when the oil is kept in a cell of small dimensions. Compared to the surrounding environment (air or other liquid tank), the reduced size of the container allows the reference oil to reach a fast equilibrium with the external unknown temperature, while the transduction of the oil temperature to optical-power variation is immediate. The field of application of this type of fiber sensors lies within the temperature monitoring in distributed environment, temperature control in large liquid tanks, etc. B. Air Relative Humidity Sensor In the case of relative humidity (RH) measurement, the aim was to find a suitable material that could readily absorb and desorb water. A widely used approach for this kind of surface sensing consists of multiple layer or polymer films. These can be prepared in a variety of ways, either by the direct polymerization of the film onto the surface, or by first preparing the polymer
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Fig. 3. Transmitted power output versus of the relative humidity for a sensor constructed with a PVA film. The inlaid plot contains data for both, increasing and decreasing cycles of humidity (power source was a 1310-nm laser).
and then coating it into the surface in a subsequent step. An example of a polymer that can form hydrogels is polyvinyl alcohol (PVA). PVA can be cross-linked by a variety of agents and can form gels containing as little as 2 to 3 wt. % PVA in water. In our case, solutions containing 3 wt. % and 4 wt. % PVA in distilled water [concentrations were expressed in weight percentage (wt.%)] were prepared to cover the fiber. Casting of the PVA films was achieved based on the application of a given amount of dissolved material on the surface of the polished fiber and letting the solvent evaporate. The experimental set up included a laser source (Fabry–Perot Rifocs 665R) to launch light into the sensor already spliced to standard FC/PC connectors and the output was led directly to a detector (Rifocs 675RE). In order to generate variations of relative humidity in a controlled way, the sensor was placed in a commercial climatic chamber (ANGELANTONI INDUSTRIE, model CH250), which was able to control the temperature and moisture of the air independently. Performed experiments included programs of ramps of increasing-decreasing water content in the air, and a computer acquired the data. The results of the measurements with this prototype are shown in Fig. 3. For a certain relative humidity range (70 to 85%), the transmission along the side-polished fiber device revealed a linear response. The coating has a higher refractive index than the fiber when dry; the guided modes thus effectively propagate through a region with a lossy cladding. Exposing the coating to water, PVA swells due to the hydrogen bonds formation with water, the film reduces its refractive index, and this analogously to the temperature sensors reduces the evanescent field penetration into the cladding inducing optical-power losses. Successive cycles rising and lowering moisture content of the climatic chamber showed almost the same results (within the experimental error values), presenting no appreciable hysteresis.
This is due to the quick time response of the sensor, faster than the moisture variation rates of the climatic chamber. In order to test in more detail this time response, specific data were taken (see Fig. 4). In this experiment, breath pulses were expelled over the sample, letting it to recover at room conditions for around one minute. Although breath is not a very reproducible event, when the same procedure was repeated over a commercial humidity sensor, the obtained values were between 85 and 95%, while room condition was around 45%. Power output response follows the values expected from Fig. 3 for this variation range, exhibiting a time needed to recover of the order of 1 min. The effect of temperature was also tested in detail for a range up to 30 C, showing no interference at all with the optical response to the presence of humidity in the air. Cross-sensitivity might well need attention at higher operating temperature which could affect the self-consistency of the polymer film. However, aiming to cover processes at room temperature the sensor was considered satisfactory.
C. Sensor of Aqueous Solutions pH Fiber-optic chemical sensors attempt to overcome the problem of selectivity by incorporating a transducer coating that is immobilized with an optically active indicator. The indicator reacts specifically with the target analyte, resulting in a modulation of the absorption (intensity) signal, while the host material provides protection, stability and in some cases even size discrimination of the diffused species. Generally, these transducers consist of polymers or sol-gel materials doped with an optical indicator that is coated on the tip or the side of an optical fiber [9], [10]. The sol-gel method was used to prepare thin films of doped porous silica matrix that were coated onto the exposed region of
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tion is not linear for the whole range, showing a low sensitivity range (from 7 to 9) separating a medium sensitivity zone, for lower values, from a very high one for values over pH 9. IV. DISCUSSION
Fig. 4. Cycles of high humidity “breaths” (indicated by on and off) to test the response in time of the sensor at 1310-nm wavelength.
the fiber. The choice of silica glass was made attending to several considerations, like inertness of the material, chemical and optical similarity to the fiber itself with close refractive indices, and good adhesion to assure long-term stability. Immobilization was the key to the development of sensors which rely on the use of functional molecules where the molecules were labile, rare or at low concentrations [11]. The silica porous matrix was both porous to analyte and still suitable for optical transduction of the recognition. Another relevant feature of the sol-gel preparation was the low temperature (close to room temperature) processing, which avoided the possible degradation of substances like the organic dye presented in this work. By means of this technique, we successfully prepared a sol-gel mixture with an organic dye to act as the sensing interface on the polished fibers. The silica solutions were prepared from the hydrolysis and posterior condensation of the alkoxide precursor, tetraethyl orthosilicate (TEOS)1 . The dye, Eriochrome cyanine R (ECR), was a highly sensitive colorant used in the determination of aluminum ions in solution which is also pH-sensitive. It was added in absolute ethanol solutions to the TEOS/water mixture to be deposited by dip coating to the polished fiber samples described before. In order to characterize the sensor, the sensing element containing the fiber was immersed into an aqueous solution and the pH was monitored simultaneously with a commercial pHmeter from Crison (GLP22). White light was launched into the single mode fiber and the absorption spectra were recorded in a UV-VIS spectrometer (halogen lamp source, model DH-2000S, and CCD type spectrometer S2000 both from Ocean Optics). The operating wavelength for monochrome experiments was chosen after the recorded spectra and it was decided to be 635 nm on account of the absorption difference for that optical frequency and the availability of a laser source. Fig. 5 contains both measurements, the spectra when the pH of the solution was varied and the posterior calibration curve at a single wavelength. The data recorded when using the laser showed excellent correlation with the absorption values in the visible spectra, proving a powerful sensitivity for pH that resulted in more than 15-dB variation for a wide range of pH values. However, this varia1All
chemical reagents were supplied by Sigma-Aldrich.
In these structures, output power change can be viewed in terms of the coupling of the evanescent field of the propagating mode with the modal conditions in the external overlay. The sensitivity of the measurement thus depends on the strength and the distribution of the evanescent field in that outer medium and the interaction is entirely defined by the refractive index of the cover material. The effect of the real part of the refractive index, given there is no absorption-induced loss, has been well established in the literature. Several authors have developed theoretical expressions of the attenuation constant in side-polished fibers [6], [12]–[14]. Different theoretical approximations as well as experimental results [3] show low level transmission attenuation for external refractive indexes (real) below the fiber core value, increasing to tenths of decibels when matching the effective index value, to recover previous transmission properties for greater refractive indices. This leads to a particular attenuation pattern depending only on the refractive index of the external medium in contact with the polished surface. This effect is responsible of the results presented for the temperature sensor. For a number of vegetal and mineral oils, the refraction index varies linearly, and significantly, with temperature [5]. Therefore, the power output of this sensor against temperature exactly reproduces the shape (symmetrically due to the negative slope refractive index variation with temperature) of the attenuation pattern previously described. The absorption in the IR region was checked and it proved to be negligible, moreover invariant within the temperature range. It can then be concluded that the oil refraction index is the only one responsible for the exhibited behavior. An appropriate selection of the oil to be used allows tailoring the design of particular temperature sensors optimally suited for different applications. The explanation of the results obtained from the humidity sensor is similar to the temperature sensor. Hygroscopic macromolecular polymers (hydrogels), in general, swell with water and the sweling lowers their refractive index [15]. This effect has already been applied to the fabrication of sensors [16]–[18]. In this work, polyvinyl alcohol (PVA) films were used, which change their refractive index as a function of the trapped water content, while they exhibit no appreciable absorption for the wavelengths used. If the real part of the refractive index remains constant for a wavelength interval, the optical power transmitted along the fiber can be sensitive to the absorption of the material where the evanescent wave expands. In other words, the absorption spectrum of the materials in the deposited layer can be observed in the signal travelling after the polished region of the fiber. This effect is the responsible of the performance of the presented pH sensor. Taking full advantage of the properties and known behavior of commonly used pH indicators, they were used as absorbing molecules within the evanescent field penetration depth (immobilised in transparent porous silica matrix readily prepared by sol-gel technique).
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Fig. 5. Calibration curve of the optical-fiber sensor with pH at 635 nm using a laser source. The inlaid shows the absorption spectra for the ECR coated fiber for pH values from 2 to 11.
After understanding the physics underlying the behavior of these sensors, the only cross sensitivity that could affect would be the effect of temperature on the humidity or pH sensors. In both cases, measurements were done showing negligible effects. Time stability was tested in some cases for periods of around one year showing again no appreciable change. These results show that the prototypes were really stable. As the polished part of the fiber is stacked in the resin block, and protected by the sensing material layer, the prototypes robustness was not a problem, being the weaker part of the pieces those bearing the unions of the fiber ends to the resin block. V. CONCLUSION In summary, we can say we have developed a series of fiber-optic sensors based on the interaction of the evanescent field in side-polished standard single mode fibers with the external medium or overlay. The technique is versatile and highly functional; in principle, any overlay material can be chosen depending on the objectives and the analyte to be measured, on the understanding that the refractive index of such material is close to the effective index of the propagating modes along the fiber. Due to the all-fiber schemes proposed, and in some case their low attenuation, the optical signal can be carried over a long distance enabling these sensors for remote sensing in hostile or hazardous environments. A family of temperature sensors has been presented. The overlay material and the region of operation can be selected to obtain a sensitivity of around 1 C over a 25 C range or 8 dB/ C over a smaller range of 5 C. A sensor of air moisture has been presented. A constant sensitivity of 0.5 dB in the range from 70% to 90% and a very quick time response (less than 1 min) are the main characteristics of this prototype.
For the presented sensor of pH of aqueous solutions, output power varies in 15 dB in the range from 2 to 11, which is a very high sensitivity over a wide range. In all the three cases, prototypes are robust and time stable, presenting negligible cross sensitivities over their operating ranges. Thus, we conclude that side polished single mode fibers are a very interesting optical structure to produce high performance sensors for different applications. REFERENCES [1] P. Hauptmann, Sensors. Principles and Applications. London, U.K.: Prentice Hall, 1993. [2] E. Udd, Fiber Optic Sensors. An Introduction for Engineers and Scientists. New York: Wiley, 1991. [3] S. M. Tseng and C. L. Chen, “Side polished fibers,” Appl. Opt., vol. 31, no. 18, pp. 3438–47, 1992. [4] R. Alonso, F. Villuendas, J. Tornos, and J. Pelayo, “New in-line opticalfiber sensor based on surface plasmon excitation,” Sens. Actuators A, vol. 37–38, pp. 187–192, 1993. [5] J. Senosiain, I. Díaz, A. Gastón, and J. Sevilla, “High sensitivity temperature sensor based on side-polished optical fiber,” IEEE Trans. Instrum. Meas., vol. 50, pp. 1656–1660, Dec. 2001. [6] O. G. Leminger and R. Zengerle, “Determination of single- mode fiber coupler design parameters from loss measurements,” J. Lightwave Technol., vol. LT-3, pp. 864–867, 1985. [7] Scheggi et al., Proc. SPIE, vol. 494, 1984, pp. 13–17. [8] G. Betta and A. Pietrosanto, “An intrinsic fiber optic temperature sensor,” in Proc. IEEE Instrumentation and Measurement Technology Conf., May 1998, pp. 1067–70. [9] B. D. Gupta and S. Sharma, “A long-range fiber optic pH sensor prepared by dye doped sol-gel immobilization technique,” Opt. Comm., vol. 154, pp. 282–284, 1998. [10] B. D. MacCraith, C. M. McDonagh, G. O’Keeffe, A. K. McEvoy, T. Butler, and F. R. Sheridan, “Sol-gel coatings for optical chemical sensors and biosensors,” Sens. Actuators B, vol. 29, p. 51, 1995. [11] R. F. Taylor and J. S. Schultz, Chemical and Biological Sensors. Bristol, U.K.: IOP, 1996. [12] C. Vasallo, “Perturbation of a LP mode of an optical fiber by a quasidegenerate field: A simple formula,” Quantum Electron., vol. 17, pp. 201–205, 1985.
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Ibon Lozano was born in Pamplona, Spain. She received the degree in telecommunication engineering from the Universidad Pública de Navarra, Pamplona, in 2002.
Ainhoa Gastón received the Ph.D. degree in telecommunication engineering from the Universidad Pública de Navarra, Pamplona, Spain, in 1993. She was a Chemist with the Universidad del País Vasco, Spain. Her work has been focused in material science and fiber-optic side polishing, mainly for sensing purposes.
Joaquín Sevilla received the Ph.D. degree in applied physics from the Universidad Autónoma de Madrid, Madrid, Spain, in 1991. He was with Westinghouse Energy Systems Spain for several years. Since 1996, he has been an Associate Professor at the Universidad Pública de Navarra, Pamplona, Spain, where teaches instrumentation for engineers. His research is mainly devoted to fiberoptic sensors.
Fátima Perez was born in Pamplona, Spain. She received the degree in telecommunication engineering from the Universidad Pública de Navarra, Pamplona, in 2000.
Fernando Auza was born in Pamplona, Spain. He received the degree in telecommunication engineering from the Universidad Pública de Navarra, Pamplona, in 2003.
